Staff at Libya's Waha Oil Company go on strike in Tripoli to protest against the sale of oil at the Es Sider port. Credit: Reuters\Ismail Zitouny

Armed rebels that have seized an oil port in eastern Libya have said that any attempt by government forces to attack a North Korea-flagged tanker loading oil at one of their occupied terminals would be like a "declaration of war".

Abb-Rabbo al-Barassi, self-declared prime minister of the movement, warned Libya's navy not to "harm" the tanker docked at Es Sider, according to a statement.

"Such a move would be a declaration of war," al-Barassi in the statement, which was sent to Reuters by a spokesperson.

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A North Korean-flagged tanker is docked at the Es Sider export terminal in Ras Lanuf. Credit: Reuters

A North Korean-flagged tanker has started loading oil at a Libyan port seized by rebels in the east of the country, port and oil officials said on Saturday.

The Libyan government has threatened to sink the tanker if it attempts to leave the port at Es Sider in Ras Lanuf. The rebels aim to sell the oil and bypass the government to gain a share of the country's considerable oil wealth.

"The loading has started," a port worker told Reuters. A spokesman for state-run National Oil Corp (NOC) confirmed this.

"The tanker will be bombed if it doesn't follow orders when leaving (the port). This will be an environmental disaster," Prime Minister Ali Zeidan told reporters, calling the docking of the tanker a criminal act violating Libya's sovereignty.

Authorities had ordered the arrest of the tanker's crew, he said. His comments were the first government reaction since the tanker reached Es Sider port, where protesters earlier said they had started exporting oil, bypassing Tripoli.